Residential houses or small industries that have a two or three phase supply of electricity have a set of mains fuses or mains circuit breakers that limits the maximum current that can be drawn from the grid as well as protects the electrical installation. FIG. 1 depicts, as an example, such an electrical installation 10 having mains fuses 11, a distribution panel 12, branch fuses 13, outlets 14 and different loads 15, 16 and 17. These reference numbers are used throughout the description for corresponding devices.
It is desired to keep the mains fuse rating as low as possible since a higher rating will result in higher fees to the grid company, and will require a larger conductor area in the supply cables. Larger conductor areas will increase installation costs for a new system and render conversions of an existing system to such larger area conductors quite expensive. However, the rating must still be high enough to enable all different load conditions without blowing a mains fuse or tripping a mains circuit breaker.
In residential houses or small industries, the current in the phase conductors varies quite rapidly due to the many different loads such as heaters, cookers, kettles, ovens, stoves, hair dryers, vacuum cleaners etc. Because of the large variation in the load, the mains fuse rating need to be set high enough not to blow during peak load conditions. However, most of the time the actual load is far below the fuse rating.
FIG. 6 shows a statistical current distribution in a typical residential house. As can be seen, 99% of the time, the current in the three phase conductors are below 16 A, although peaks up to 25 A occurs. The mains fuses and the supply cables must thus be rated to at least 25 A.
Thus, there is a tradeoff between selecting a small mains fuse rating for cost reasons, but the rating should still be high enough to prevent blowing a fuse during peak load conditions.
A solution to this problem is provided by load shedding switches. They are current limiting devices that sense the supply phase currents and cuts off a preselected load if the supply current exceeds the mains fuse rating. This is for example described in SE 515 272 and CH 641 295.
Disadvantages with these methods are that the disconnected load obviously can not be utilised during the time it is disconnected, and a further problem is that the load that is disconnected must either be a three phase load or a load on the same phase that is currently exceeding the mains fuse rating. In many households/small industries there are not many three phase loads that directly can be disconnected. Further many loads are plug-connected movable equipment while traditional load shedding switches only work with permanently installed equipment.
Further, modern homes/industries are getting more and more single phase loads, since there has been a propensity for manufacturers to move to such single phase devices for all but the most power consuming devices.
Many products on the market are available for the purpose of limiting the supply phase currents below the capacity of the mains fuses or the mains circuit breaker. They all work by:
a) Disconnecting preselected loads during phase current peaks, as described above. Here loads are disconnected using mechanical relays in order to prevent an exceeding of the mains fuse rating. Refer to FIG. 2 for a diagrammatic outline of such a system. Relays 22 can be used to shed or disconnect loads 16, 17 when any electric current through any fuse/circuit breaker 11 approaches the rating of the same.
or
b) Switching a load from one phase to another
This is for instance described in U.S. Pat. No. 5,181,180 Munro, and by U.S. Pat. No. 7,242,110 Matsumoto et al. These references describe the use of relays to disconnect loads or transfer loads between phases. Refer to FIG. 3 for a diagrammatic outline of such a system. Relays 22 can be used to transfer loads 17 from one phase conductor to another when any electric current through any fuse/circuit breaker 11 approaches the rating of the same.
Both these methods address the problem of overload of a supply conductor but nevertheless introduce some disadvantages. For instance, when a load is transferred there will be some electric transients due to the phase shift between the phase conductors. Such transients may shut down sensitive electronic equipment.
Further, the system must have knowledge about which loads may be transferred or disconnected, i.e. careful planning is required to preselect loads for disconnection and loads may need to be connected to different phases to balance consumption. Also, despite such (often laborious) planning of an electrical installation, there will be a poor function in systems with many portable loads, since the magnitude and placement of such movable loads cannot be anticipated and therefore not taken into account for. Of course, for any disconnected load, such loads cannot be utilised during the time they are disconnected.